This invention relates to apparatus and method for detecting far end speech.
In telephone systems designed to operate with a half duplex communications system, the outgoing speech signal from the near end telephone apparatus is muted so as not to be transmitted to the far end user when the far end user is speaking. Otherwise, the far end speech is muted, at the near end telephone apparatus. This may be effected by detecting at the near end a condition where the received far end speech signal is of significant magnitude, and, if so, effecting muting at the near end of the outgoing near end speech signal, the far end speech signal at the near end telephone apparatus otherwise being muted.
Telephone apparatus for implementing the above generally employs, internally, four wire signal transmission. That is to say, the signals from and to the telephone microphone and loudspeaker respectively are on separate communications paths, this being necessary to enable near end and far end signals to be distinguished as necessary for performing half duplex operation. The telephone apparatus may thus incorporate a hybrid the purpose of which is to couple the two and four wire signal paths. At the microphone and loudspeaker, the signals are in analogue form. Transmission over the communications link between the near and far end telephone apparatuses will normally be effected by analog signals over a two wire link, such as a Public Subscriber Telephone Network (PSTN). Internally, each telephone apparatus may incorporate some means for converting to and from digital and analogue signal formats.
If the impedance characteristics of the hybrid provide exact signal matching, outgoing near end signal, from the near end microphone, is directed entirely to the two wire link. However, if there is imperfect matching, some part of the near end signal from the near end microphone will pass through the hybrid to be mixed with incoming far end signal on the signal path to the speaker of the near end telephone apparatus. Practically, it is impossible to ensure exact matching, so there is always some returned signal of this kind, called “sidetone”.
The existence of sidetone signal presents a particular problem in half duplex signal transmission, because it can interfere with the ability to detect, at the users' telephones, when the far end user is or is not speaking, with the consequence that conditions requiring muting or non-muting of speech signals are not properly detected. A particular problem of this nature occurs where the near end telephone is in an environment where there is an acoustically noisy background. In this case, the side-tone signal may be of significant magnitude, such that detection of a condition where the far end user is not speaking is not properly effected because there is sufficient sidetone signal mixed with the incoming far end speech signal to initiate muting of the outgoing near end speech signal, regardless of whether or not the far end user is speaking, it being impossible to distinguish the sidetone from the far end user's speech signal. That is, at the near end telephone, side-tone signal is detected and interpreted as indicating a condition where the far end user is speaking, so that the far end signal is, perhaps constantly, muted at the near end user's telephone. Of course, in this circumstance, communication between the parties is interfered with and may be rendered impossible.